Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver to improve communication performance. MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the same total transmit power over the antennas to achieve spectral multiplexing that improves the spectral efficiency (more bits per second per Hz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading), or increased antenna directivity.
The term “beamforming” sometimes referred to as “spatial filtering” as used herein, is a signal processing technique used in antenna arrays for directional signal transmission or reception. This is achieved by combining elements in the array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity.
The term “beamformer” as used herein refers to RF circuitry that implements beamforming and may include a combiner, switches controllable phase shifters, and in some cases amplifiers.
The term “Receiving Radio Distribution Network” or “Rx RDN” or simply “RDN” as used herein is defined as a group of beamformers as set forth above.
The term “Spatial Multiplexing” as used herein applies to the technique where different MIMO signals streams are transmitted from multiple antennas. Each of these signals is from a set of data streams that is transmitted in a manner (e.g., different pre-coding) to ensure low channel correlation among data streams at the receiver Spatial Multiplexing may be employed in conjunction with beamforming.
The term “autonomous” as used herein describes a process that is performed by one side alone (e.g., the transmit side or the receive side of a communication system), without supporting signaling or feedback from the other side.
The term “collaborative” as used herein describes a process that uses cooperation between both sides of a communication link to assist each other. (e.g., in a communication system, the base station and the user equipment exchange information to assist each other in improving the link).
The term “blind phase scan” as used herein, is an autonomous signal quality enhancing technique, according to which the phase of the receiving antennas is methodically changed while simultaneously monitoring one or more preselected quality indicators such as power control, SINR, Signal to noise ratio (SNR), or some cases a data rate measurement. The phase parameters are periodically tuned and updated so as to optimize the preselected one or more quality indicators.
The term “Maximal Ratio Combining” or “MRC” as used herein, is an autonomous signal quality enhancing technique based on Diversity combining, in which the signals from each channel are added together, and the gain of each channel is made proportionally to the RMS signal level and inversely proportional to the Mean Square noise level.
The term “Optimal Combining” or “OC” as used herein, is an autonomous signal quality enhancing technique based on Diversity combining, in which the signals from each channel are combined together to maximize Signal to Interference plus Noise Ratio (SINR).
The term “Least Mean Squares” or “LMS” as used herein, is an autonomous signal quality enhancing technique in which an equalizer filter processes a signal derived from signals received by a plurality of antennas. In some cases, a filter coefficient correction used by the equalizer filter may be generated by a tap coefficients generator using a least mean square (LMS) algorithm.
The term “interference cancellation” as used herein, is an autonomous signal quality enhancing technique based on selectively removing or reducing undesired interference, in such a way that improves SINR of the desired signal.
The term “downlink transmit beamforming” as used herein, is a collaborative signal quality enhancing technique based on signaling between user equipment (UE) and base transmitter station (BTS), in which the BTS is provided with information received by the UE, regarding the desired tuning of its DL beamforming weights, e.g., using pilot signals coming from the BTS, and the UE sends feedback informing the BTS of desired corrections to be applied to its DL antennas' weights. This MIMO scheme is also referred to as Closed Loop BF.
The term “minimum mean-squared error” or “MMSE” as used herein, is a process for cases where a digital radio-communications systems operating on a jammed frequency-selective fading channel: The receiver performance can be improved by using the joint antenna diversity and equalization techniques to combat both time- and frequency-selective fades and jammers effects. In this process, the optimum, in the sense of MMSE, the structure of the linear equalizer (LE), and the decision feedback equalizer (DFE) for coherent receiver antenna diversity are all being derived for an un-jammed environment.
The term “Transmit Diversity” as used herein, sometimes called “Alamouti Tx Div” refers to a collaborative signal quality enhancing technique, where L transmitting antennas simultaneously emit up to L consecutive symbols, in up to L combinations, so that each given symbol is repeated up to L times, yielding time diversity without sacrificing bandwidth.
Many techniques are known in the art for enhancing signal quality in RF MIMO communication systems. The aforementioned techniques are a mere few and other techniques, currently the RF MIMO signal quality enhancement methods are implemented in the baseband domain, by a baseband DSP module.
FIG. 1 is a high level schematic block diagram illustrating a MIMO receiver system 10 in accordance with the prior art. A baseband DSP processor 20 is fed by two or more radio circuits 30-1 to 30-N, each of which is in turn fed by its respective antenna 40-1 to 40-N. In operation, baseband DSP processor 20 may apply one or more signal quality-based enhancement techniques, including autonomous or collaborative techniques, or both, that may include, but are not limited to, the techniques discussed above.
There are several issues associated with the aforementioned architecture: Firstly, 3GPP standardization supports several canonical MIMO configurations, e.g., 2×2, 4×4, or 8×8, and consequently, protocols, base stations' software, and UE DSP software products do not currently support a flexible number of UE antennas. Second, the more complex standard configurations (e.g., 8×8) are going to take a while before they are brought to market. Third, the more complex standard configurations would be expensive, since advanced UEs need to support many RF bands (e.g., 7) and when the number of antennas is increased by a factor (e.g., by 1:5), then the RF chains supporting it must grow by such a factor, e.g., from 14 (i.e., 2×7) to 70 (i.e., 10×7), which becomes exceedingly expensive.